For determining proof of alcohol percent, all hydrometers are made according to the Bureau of Internal Revenue, US Treasure Department. The tolerances listed for these hydrometers are those outlined in Circular 555 of the National Bureau of Standards, Testing of Hydrometers. Each hydrometer is calibrated in a triple accredited ISO/IEC 17025:2005, A2LA accredited laboratory against equipment whose calibration is traceable to NIST. Individually packaged with a multi-language Statement of Accuracy indicating accuracy traceable to NIST.